1. Field of the Invention
The present invention relates to a method of fabricating semiconductor devices and, more particularly, to a method of forming fine patterns of semiconductor devices.
2. Description of the Related Art
There is currently a great amount of research in methods for forming fine patterns of semiconductor devices and photomasks necessary therefore because semiconductor devices are becoming more highly integrated. Presently, a phase shift mask process is widely used to form fine patterns of semiconductor devices because it is highly effective in forming periodic patterns. However, elaborate techniques are required to form a phase shift mask, and it is expensive to fabricate the phase shift mask, moreover, phase shift mask processes do not effectively form aperiodic patterns. Thus, processes using a phase shift mask are inappropriate for forming fine aperiodic or abnormal patterns at low cost. Since, pattern density increases as integration increases and pattern abnormalities increase as pattern density increases, pattern abnormalities in semiconductor devices increase with the increased integration thereof.
FIG. 1 is a plan view partially showing a conventional photomask 1. Here, reference numerals 5 and 7 denote a cell array region having high pattern density and a peripheral circuit region having low pattern density, respectively.
Referring to FIG. 1, adjacent contact hole patterns 3 are two-dimensionally separated by a predetermined distance d in the cell array region 5. On the other hand, contact hole patterns 3 in the peripheral circuit region 7 are two-dimensionally separated by a distance which is wider than the predetermined distance d.
FIG. 2A is a cross-sectional view of the photomask 1 shown in FIG. 1 taken along line A-Axe2x80x2. FIG. 2B is a graph of the light intensity of light beams passing through the contact hole patterns 3 of the photomask 1 shown in FIG. 2A. FIG. 2C is a cross-sectional view of a profile of a photoresist pattern 26 formed in accordance with the light intensity of the graph shown in FIG. 2B. Here, reference characters 5 and 7 denote the cell array region 5 and the peripheral circuit region 7 of FIG. 1, respectively.
Referring to FIGS. 2A, 2B and 2C, an interdielectric layer 23 is formed on a semiconductor substrate 21, and an etch mask layer 25 having a high selectivity with respect to the interdielectric layer 23 is formed on the interdielectric layer 23. A photoresist layer 26 is formed on the etch mask layer 25, and the photoresist layer 26 is exposed to ultraviolet light using the photomask 1 and light 20 shown in FIG. 2A. The photomask 1 is composed of a transparent substrate 22 and an opaque material pattern 24 formed in a predetermined region on one side of the transparent substrate 22. The opaque material pattern is made of a chrome pattern. The opaque material pattern 24 is formed in the regions between the plurality of contact hole patterns 3 shown in FIG. 1. That is, the opaque material pattern 24 is formed by removing material or preventing the formation of material where the contact hole patterns 3 are transcribed thereon, thereby defining holes where the contact hole patterns are transcribed.
As described above, light 20 passes through the transparent substrate 22 and through the contact hole patterns 3 of the opaque material pattern 24 when the light 20 radiates the photomask 1. The light which passes through the transparent substrate 22 and the contact hole patterns 3 exposes a predetermined region of the photoresist layer 26 formed on the semiconductor substrate 21. The diffraction or interference effect on the light beams passing through the transparent substrate 22 and contact hole patterns 3 is more severe as the distance d between adjacent holes in the contact hole pattern 3 decreases. The etch mask layer 25 is formed on the interdielectric layer 23 and acts as an antireflection layer, but the diffraction and interference effects cannot be overcome.
Thus, as shown in FIG. 2B, light beams irradiate some of the region under the opaque material pattern 24. As a result, the portion of the photoresist pattern 26 corresponding to the contact hole patterns 3 formed in the cell array region 5 are inaccurately formed. Thus, when the etch mask layer 25 and the interdielectric layer 23 are etched using the photoresist pattern 26 as a mask, contact holes having an inaccurate profile are formed in the cell array region 5.
In the conventional art, when the sizes or shapes of patterns of the photomask 1 are different, it is difficult to optimize profiles of all patterns formed on the semiconductor substrate. For example, when patterns having various sizes or shapes are mixed in the photomask 1, it is difficult to optimize profiles of all patterns. This is because conditions of the photo process, for example, conditions of exposure and development, must be changed in accordance with the size or the shape of the pattern.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.
In accordance with one aspect of the present invention there is provided a photomask for forming fine patterns capable of optimizing the profiles of all patterns formed in a high pattern density region. More specifically, there is provided a photomask for forming fine patterns by etching one material layer formed on a semiconductor substrate. The photomask for forming fine patterns includes at least two sub-photomasks. Each of the sub-photomasks is composed of one transparent substrate and patterns on one side of the transparent substrate. All of the patterns to be formed can be thought of as an entire set of patterns which are split into subsets of patterns. The union of the subset of patterns is the entire set of patterns. Each sub-photomask is transcribed with a corresponding subset of patterns. The patterns of one sub-photomask correspond to one of the groups obtained by classifying a group of patterns finally formed on the semiconductor substrate. For instance, when the photomask is composed of two sub-photomasks, patterns on a substrate of the first sub-photomask (i.e., patterns of a first group) are patterns obtained by transcribing some of the entire set of patterns. The patterns on a substrate of the second sub-photomask (i.e., patterns of the second group) are patterns obtained by transcribing the entire set of patterns with the exception of the patterns of the first group.
In accordance with another aspect of the present invention, at least one of the patterns on one sub-photomask may overlap with at least one of the patterns on another sub-photomask. The overlapped pattern may correspond to a pattern such as an alignment key, which is difficult to completely pattern with one photo process.
In a further aspect of the present invention, when the sizes of the patterns in the entire set of patterns are the same, the minimum distance between patterns on the substrate of each of the sub-photomasks is wider than the minimum distance among the entire set of patterns. Thus, when the photo process is performed using each of the sub-photomasks, the distance between patterns is increased, so that the interference and/or diffraction effect of light beams can be remarkably suppressed.
In accordance with yet another aspect of the present invention, when there are two different shapes of patterns in the entire set of patterns, patterns of the same shape are transcribed on a substrate of one sub-photomask. For instance, when the entire set of patterns are classified into two kinds of shapes, i.e., rectangular patterns and regular square patterns, the rectangular patterns are transcribed on the substrate of one sub-photomask and regular square patterns are transcribed on the substrate of another sub-photomask. Here, the rectangular pattern may be a pattern for forming an interconnection of a bar type or a pattern for forming an oval contact hole, and the regular square pattern may correspond to a pattern for forming a circular contact hole. Thus, it is easy to optimize the profiles of the regular square patterns and rectangular patterns formed on the semiconductor substrate by changing the process conditions whenever a photo process using each of sub-photomask is performed.
In accordance with another aspect of the present invention there is provided a method of forming fine patterns using the photomask. More specifically, there is provided a method of forming fine patterns capable of optimizing the profiles of all patterns formed on a semiconductor substrate. In this method, one material layer to be patterned such as a dielectric layer or a conductive layer is formed on the semiconductor substrate, and the material layer is patterned using at least two sub-photomasks. Here, the sub-photomasks are the same as those of the sub-photomask for forming fine patterns. As a result, the method of forming fine patterns requires the same number of photo processes as the number of sub-photomasks.
In a further aspect of the present invention, when two sub-photomasks are used (i.e., first and second sub-photomasks) the step of patterning the material layer includes the sub-steps of forming a first photoresist pattern on the material layer using the first sub-photomask; first-patterning the material layer using the first photoresist pattern as an etch mask; removing the first photoresist pattern; forming a second photoresist pattern using the second sub-photomask which is different from the first sub-photomask on the semiconductor substrate where the first photoresist pattern is removed; patterning the first patterned material layer using the second photoresist pattern as an etch mask, thereby forming a plurality of patterns in the material layer.
In accordance with yet another aspect of the present invention, the present invention may further comprise a sub-step of forming an etch mask layer having an etch selectivity with respect to the material layer. At least two of the sub-photomasks are used for patterning the etch mask layer. Also, the material layer is patterned by one etching process using the patterned etch mask layer as an etch mask, thereby forming desired patterns. The etch mask layer is formed of a silicon nitride layer or a silicon oxynitride layer acting as an antireflection layer. When the two sub-photomasks are first and second sub-photomasks, the step of patterning the material layer includes the sub-steps of: forming the first photoresist pattern on the etch mask layer using the first sub-photomask; patterning the etch mask layer using the first photoresist pattern as an etch mask; removing the first photoresist pattern; forming a second photoresist pattern on the semiconductor substrate where the first photoresist pattern is removed; using the second sub-photomask which is different from the first sub-photomask; second-patterning the first-patterned etch mask layer using the second photoresist pattern as an etch mask; removing the second photoresist pattern, and etching the material layer using the second-patterned etch mask layer.
In accordance with still another aspect of the present invention, when the shapes of the patterns of the first sub-photomask are different from that of the second sub-photomask, the photo process using the first sub-photomask is performed under different conditions than that of the photo process using the second sub-photomask, thereby easily optimizing the profiles of all patterns finally formed in the material layer. When all patterns finally formed in the material layer have the same size, the profiles of all patterns formed in the material layer can be optimized by manufacturing first and second sub-photomasks such that a minimum distance of patterns transcribed on the first sub-photomask and a minimum distance of patterns transcribed on the second sub-photomask are wider than the distances of the plurality of patterns. This is because whenever photo processes are performed using each of the sub-photomasks, the minimum distance of the patterns transcribed on the semiconductor substrate is wider than that in the conventional art, so that the interference or diffraction effect of the light beams generated between adjacent patterns is remarkably reduced. Particularly, when an etch mask layer is additionally formed on the material layer, profiles of each of the patterns can be further optimized.
According to the present invention, a plurality of patterns which are formed by using single photo process, are formed by using at least two photo processes, thereby optimizing the profiles of each of the patterns.